Voltage mode buck regulators have become commonplace in today's broad market. Many of these regulators require external compensation. External compensation allows the regulator to be used with a wide range of components and gives the user control over the systems' performance. While the user has considerable control over the system, many find themselves lacking the proper resources to take advantage of this feature. Part 1 provides the criteria, output filter analysis, understanding of compensation networks, and procedures to compensate a switcher.
This paper will discuss the technical motivation behind compensation, derivation of analytical and design oriented transfer functions, establish criteria for the selection of a particular type of network, and provide an illustrative example. This part offers a series of 'quick sheets' and appendices that are a useful reference in the design of compensation networks.
Consideration must be given to the size of components in a physical implementation. For example, capacitors should not be so small that parasitic trace components become large compared to the component value. Capacitors should also not be so large that they draw an excessive amount of current from the error amplifier, limiting the slew rate. Ceramic capacitors should be used with typical values in the 10pF to 5nF range.
Similar considerations apply to the selection of resistor values. Small resistor values may draw an excessive amount of current from the error amplifier, while large resistor values may be susceptible to noise. Typical values are in the 1kΩ to 100kΩ range.
One should not become concerned with being entirely exact in the calculation of component values. The models used do not account for all stray LCR elements that vary from layout to layout. If the loop response is measured, it will likely differ from the actual response. This is due to the disturbance of the loop necessary to measure the response.
Figure 1: Switcher output network
Figure 2: Output network algebra on the graph. Z refers to the complex impedance of an element.
Figure 3: Bode phase plot approximation